CQDs are semiconductor particles only a few nanometres in size. They can be synthesized in solution, which means that films of the particles can be deposited quickly and without fuss on a wide range of flexible or rigid substrates – just like paint or ink can.

CQDs could be used as the light-absorbing component in cheap, highly efficient inorganic solar cells. In a solar cell, high-energy photons hitting the photovoltaic material can produce excited electrons and holes (charge carriers) that have energies at least equal to or greater than the band gaps of the material. Another advantage of using CQDs as the photovoltaic material is that they absorb light over a wide spectrum of wavelengths thanks to the fact that the band gap can be tuned over a large energy range by simply changing the size of the nanoparticles.

There is a snag, however; the high surface area to volume ratio of nanoparticles results in bare surfaces that can became “traps” in which electrons invariably get stuck. This means that electrons and holes have time to recombine instead of producing useful current, something that inevitably reduces the efficiency of devices made from CQD films.

Surface passivation

A team led by Edward Sargent at Toronto may now have come up with a solution to this annoying problem. The researchers have succeeded in passivating the surface of CQD films by completely covering all exposed surfaces using a chlorine solution that they added to the quantum-dot solution immediately after it was synthesized. “We employed chlorine atoms because they are small enough to penetrate all of the nooks and crannies previously responsible for the poor surface quality of the CQD films,” explained Sargent.

The team then spin cast the CQD solution onto a glass substrate that was covered with a transparent conductor. Next, an organic linker was used to bind the quantum dots together. This final step in the process results in a very dense film of nanoparticles that absorbs a much larger amount of sunlight.

Reducing traps

“Our hybrid passivation scheme employs chlorine atoms to reduce the number of traps for electrons associated with poor CQD film surface quality while simultaneously ensuring that the films are dense and highly absorbing thanks to the organic linkers,” Sargent told nanotechweb.org.

Electronic spectroscopy measurements confirmed that the films hardly contained any electron traps at all, he adds. Synchrotron X-ray scattering measurements at sub-nanometre resolution performed by the scientists at KAUST corroborated the fact that the films were highly dense and contained very closely packed nanoparticles.

Low-cost photovoltaics on the horizon

“Most solar cells on the market today are made out of heavy crystalline materials,” explained Sargent, “but our work shows that light and versatile materials like CQDs could potentially become cost-competitive with these traditional technologies. Our results also pave the way for low-cost photovoltaics that could be fabricated on flexible substrates, for example, using roll-to-roll manufacturing (in the same way that newspapers are printed in mass quantities).”

The team is now looking at further reducing electron traps in CQD films for even higher efficiency. “It turns out that there are many organic and inorganic materials out there that might well be used in such hybrid passivation schemes,” added Sargent, “ so finding out how to reduce electron traps to a minimum would be good.”

The researchers say that they are also interested in using layers of different-sized quantum dots to make a multi-junction solar cell that could absorb over an even broader range of light wavelengths.

The current work is detailed in Nature Nanotechnology.